Grain has been ground to make flour for ages. Early grinders used water, wind or horse power to turn one milling stone against another. Commonly ground grains include wheat, barley and rye, but corn, rice, beans and numerous other foodstuffs can be similarly ground.
More recent grinders use electrical power to turn a grinder in a higher speed process. In particular, electrical grinders have been used to grind grain for testing. One particular prior art grain testing mill, commercially available as a PERTEN 3100 mill, grinds grain using a high speed hammer type impeller in a screened (0.8 mm openings) chamber. The impeller is driven at about 16,800 rpm, and grinds about a 300 g sample in about 30-50 seconds. The grinding process utilizes the air stream created by the impeller to move the grain material about during grinding and to carry the flour away from the grinding chamber to a filter bag. If the mills grind the grain in a sufficiently fine, relatively homogeneous particle size, the ground particles can be used for sprout damage testing, viscosity analysis, toxin testing, pesticide testing, genetically modified organism (GMO) testing, falling number (FN) testing, Glutomatic/Gluten Index testing, near infrared (NIR) testing, stirring number (SN) testing and other tests. These tests can be performed on grain in grain receival, storage and export silos, elevators, terminals and grain laboratories.
When grain is ground at high speed, it creates not only the particles which make up the flour, but also smaller particles which remain airborne as dust even after the air stream from the impeller loses speed. Generally, to avoid the release of dust into the room where people are breathing and to avoid the mess created when the dust settles, the air stream from an impeller grinder is filtered such as through a filter bag or a separate filter prior to release to ambient. While the filtration process may effectively remove dust from the air, the grain dust particles often build up on the filter. The filter should be regularly cleaned or changed, resulting in additional costs of maintaining the grinding mill. If the filter is not regularly cleaned or changed, grain dust buildup on the filter can restrict airflow and adversely affect grinding mill operation. A fast grinding process which could avoid the creation and build-up of dust, and which could avoid the expense and hassle of filter changes, would be beneficial.
The grinding process also generally releases moisture (water molecules) and other smaller-than-dust particles into the air. The PERTEN 3100 mill, for example, typically dries the sample by approximately 0.5-1.0%, so the sample is not suitable for direct moisture testing, and moisture corrections need to be made for FN and SN testing. Depending upon the use of the ground material, it may be desired to quickly grind the material without releasing as much moisture or other smaller-than-dust particles.
Another problem with grinding mills, particularly when the resultant ground product is used for testing, is cleaning of the mill and carry-over of residual material from one milled sample to the next. Any mixing of samples decreases accuracy of the sample and the testing results. To the extent possible, a grinding mill should be easy to clean and should minimize carry over of residual material. Typical time required to sufficiently clean a PERTEN 3100 mill to eliminate carry-over between samples is 4½ minutes (includes cleaning of the grinding chamber, sample flow tube and cyclone).